As an acoustic wave device utilizing an acoustic wave, there is known a surface acoustic wave (SAW) device that includes an interdigital transducer (IDT) including a pair of comb-like electrodes on a piezoelectric substrate, and a pair of reflectors between which the IDT is interposed on the piezoelectric substrate. The SAW device is used in bandpass filters in various circuits that process wireless signals in the frequency range of 45 MHz to 2 GHz, which is typically used in cellular phones. In recent years, with improvements in the performance of wireless communication devices, which are typically cellular phones (for example, multi-band or multi-mode performance), it has been demanded to reduce the insertion loss of the filters for the purpose of improving the receiver sensitivity of the wireless communication devices and reducing the power consumption.
There have been various proposed acoustic devices intended to reduce the power consumption of the filters. For example, there has been a proposed structure in which part of a bus bar is thicker than electrode fingers (see Japanese Patent Application Publication Nos. 2002-100952 and 2004-328196, for example). Another structure has been proposed in which a protection film that covers the IDT is relatively thick except a part in which electrode fingers are interleaved (see Japanese Patent Application Publication No. 2007-110342, for example). There has been yet another proposal of widening end parts of electrode fingers or adding a metal film or the like to end parts (see Japanese Patent Application Publication No. 2012-186808, for example). There has been a further proposal of a structure in which an insulator is buried between an electrode finger and a bus bar or between an electrode finger and a dummy electrode (see Japanese Patent Application Publication No. 2009-278429, for example).
However, in the conventional acoustic devices, there is yet room for improvement in the reduction of the insertion loss.